1. The Field of the Invention
The present invention relates to lead and arsenic free optical hard crown glasses with a low transformation temperature (Tg≦520° C.), characterized by their optical range with a refractive index of 1.57≦nd≦1.61 and an Abbe number of 56≦νd≦63.
2. The Description of the Related Art
In the recent years the tendency of the market in the field of optical technologies as well as opto-electronic technologies (application areas mapping, projection, telecommunication, optical communication engineering, optical data processing and laser technology) goes more and more into the direction of miniaturization. This is observable with the finished products and therefore appears also with single structural members and components. For the producers of optical glasses, this means first of all a clear decrease of the demanded volumes in spite of increased quantities of products.
Furthermore, there is an increasing pricing pressure on the side of the re-processors, because more waste noticeably accumulates with clearly lower sizes of the components made of block/ingot glass procentually related to the product and in addition the processing of such miniature parts causes a highly increased operating expense.
For that reasons, the re-processors' requests for blank pressed small components and also for their preliminary stages, namely for preforms close to the final geometry for repressing, the so-called “precision gobs” or, shortly, “gobs”, increase. These precision gobs are completely fire-polished, free or half-free formed glass portions, the production of which is imaginable by various ways.
One production method for gobs is the beaded spray procedure. In this connection no selective portioning is necessary, and the remaining fraction doesn't have to be wasted after the separation of the specific size fraction (e.g. sieving), but can be recycled as highly pure and particularly well re-melting cullets. Furthermore large numbers of parts are realized within a short period of time using that procedure, which is technically and personally very easy to handle.
In contrast to that, direct pressing of parts close to the final geometry, which has to be judged as being more laborious and more difficult, raises the problem of profitability. This procedure can comply with the smaller volumes of melted glass (distributed on a high number of small parts of material) in a flexible way by shorter set-up times but with the small geometries the creation of value cannot be caused by the value of the material alone because of the smaller number of parts per time unit compared with the “gob spraying”. Rather, the products should leave the press in a state ready for installation, without laborious post-processing, cooling and/or cold re-processing. Because of the required high accuracy of geometries, precision instruments with high grade and therefore expensive mold materials must be used. The lifetimes of the molds massively affect the profitability of the products/materials.
A very important point concerning lifetimes is the working temperature, which conforms with the viscosity of the materials to be pressed. This means for glass as a material, that the profitability and therefore the earnings increase the lower the transformation temperature (Tg) of the glass is. Thus, there is a demand for so-called “low-Tg-glasses”, i.e. glasses with a low transformation temperature, which also means low melting temperatures and low temperatures at the corresponding viscosity points during processing.
Further, from a process technological point of view, the melting behaviour of the glasses is of some importance, because there is a growing demand for so-called “short” glasses.
The viscosity of those glasses changes strongly with the temperature. An ideal behaviour concerning the change of viscosity in dependence on the temperature is of that kind, that a glass is indeed clearly “shorter” (which means a stronger change of viscosity in dependence on the temperature) than e.g. classic BK7, but not so extremely “short” like the glasses of the family of lanthanum borate glasses. An optimised behaviour of that kind has the advantage in the process, that the times of hot forming, i.e. the closure times of the mold, can be decreased because of the adjusted behaviour of the glass. Because of that, the throughput (reducing of the cycle times) can be increased and further the material of the mold is conserved at the same time, which has clearly a positive effect on the costs of the whole production.
Another aspect is, that glasses with adjusted behavior concerning the viscosity in dependence on the temperature can also be cooled faster, which offers the possibility of processing glasses with a higher tendency for crystallization without prenucleation, which could cause problems in succeeding steps of secondary hot forming. This aspect results in the fact, that suitable glasses are not only suitable for the producing of “gobs” and for direct pressing, but further also for the fibre production, which the corresponding glasses have in common with lanthanum borate glasses.
The prior art describes glasses, which all have immense disadvantages:
JP 63170247 A describes glasses with a gradient of the refractive index. The mentioned contents of alkali oxides are responsible for a strong widening of the intrinsically stable network structure and therefore for a large mobility of the single components. This is volitional and is used specifically for the purpose of quick and effective ion exchange. The mobility of the nucleating and crystallizing agents conclusively rises with the mobility of the fluxing agent, which leads to an enhancement of the tendency of nucleation in a primary process of hot forming as well as to an enhanced crystal growth in a secondary process of hot forming. Properties like that are not desired and can be avoided by defined low contents of the two main nucleating agents of that system, TiO2 and ZrO2, but which is not suggested by that document of the prior art. Rather, for example the content of TiO2 can be up to 32 wt %. Further there is no hint in JP 63170247 A, that arsenic oxide or lead oxide should be avoided in the glasses. The use of zinc oxide as a component is not suggested.
GB 2 233 781A describes ophtalmic glasses with low Abbe numbers of 40 to 54. Again, high contents of the main nucleating agents TiO2 und ZrO2 (total amount 5.7-33.8 wt %) are used, which is accompanied by massive disadvantages by the previously mentioned reasons (keyword: crystallization). Further, that document of the prior art discloses glass compositions with high amounts of alkali oxides, namely at least 30 wt %, which again leads to the mentioned disadvantages, namely, that the mobility of all components rises so far, that the tendency of crystallization is drastically increased.
JP 05017176 A also describes glasses with low Abbe numbers (νd 30-55). Those numbers are achieved as well by the use of up to 20 wt % TiO2 and ZrO2, which leads to the known disadvantages.
The same can be said for the glasses, which are disclosed in JP 06107425 A. Their Abbe number is at most 27. Again, the content of TiO2 and ZrO2 can be in total up to 40 wt %, which leads to undesired tendency of crystallization.
JP 08012368 A describes potentially lead containing glasses (PbO<5 wt %) for pressing. The use of Lithium oxide in the glasses is obligatory in high amounts from 7 to 12 wt %, which gives even more disadvantages to the material: first of all, Li2O, as an extremely potent fluxing agent, strongly increases the mobility of the glass components with the consequence, that nucleation and crystallization are favored. Then, the use of Li2O in combination with B2O3 (5-30 wt %) leads to an increased damage of crucibles, which has a strong and undesired transmission decreasing effect in the case of platinum aggregates.
The glasses, that are disclosed in U.S. Pat. No. 5,744,409 and contain also Li2O contents between 7 and 12 wt %, show comparable disadvantages. Further those glasses can contain up to 5 wt % PbO, which is undesired from an ecological point of view.
WO 02/96818 A1 describes crystallized glass with an unfavorably high content of nucleating agents (TiO2+ZrO2 up to 28 wt %) and alkali oxides (up to 84 wt % with K2O>3.7 wt %), which leads to a homogeneous and numerous nucleation and crystallization as a basis for homogeneous material. But the crystallization is not desired in the areas of the above mentioned applications.